ModelGenerator: add option to create just custom fields

[tone711]

Add a new option to the ModelGenerator, "-use-base-models", that incorporates the existing NetCoreForce.Models.

For example, when creating the object "Account", with the prefix "MyCustom" and the new "-use-base-models" flag, it creates the class like doc example.

public class MyCustomAccount : SfAccount
{
    public string MyCustomField__c { get; set; }
    public string MyOtherField__c { get; set; }
}